A Usable, Secure Communications Protocol: Client-Side TLS - Implementing SSL/TLS Using Cryptography and PKI - page 347

Computing the Verify Message

To compute this verify message, then, it's necessary to keep a running hash
of every byte that's sent or received with a message type of handshake . This is,
incidentally, why I spent so much time in Chapter 4 on creating an “updateable”
HMAC function; without the updateable HMAC function, it would have been
necessary here to buffer all this data and pass it as a gigantic memory array
into the HMAC function.
Instead, following these steps:

1. Add a pair of digest_ctx objects to the TLSParameters as shown in Listing 6-46;
the verify data is actually based on a combination of both MD5 and SHA
(similar to the PRF).

Listing 6-46: “tls.h” TLSParameters with digest contexts

typedef struct
{
…
int server_hello_done;
digest_ctx md5_handshake_digest;
digest_ctx sha1_handshake_digest;
}
TLSParameters;

2. At the top of tls_connect , initialize them both, in Listing 6-47.

Listing 6-47: “tls.c” tls_connect with handshake digests

int tls_connect( int connection,
TLSParameters *parameters )
{
init_parameters( parameters );

new_md5_digest( &parameters->md5_handshake_digest );
new_sha1_digest( &parameters->sha1_handshake_digest );

3. Modify send_handshake_message , as shown in Listing 6-48, to update the
running digest every time a handshake message is sent.

Listing 6-48: “tls.c” send_handshake_message with handshake digest update

static int send_handshake_message( int connection,
int msg_type,
const unsigned char *message,
int message_len,
TLSParameters *parameters )
{
...
update_digest( &parameters->md5_handshake_digest, send_buffer,
send_buffer_size );

(Continued)